This invention relates to radiation-curable coating compositions for coating electrical conductors, and in particular, to coating compositions which provide good insulating properties in extreme conditions.
Electrical conductors are in general coated with a dielectric coating for insulating the conductor. Such coatings require good insulating properties in a wide variety of application environments including, for example, microelectronics applications such as, for example, semiconductors, printed circuit boards, capacitors and resistors, and heavy industrial applications such as, for example, motors, coils, generators and transformers and electrical wiring systems. In certain applications, for example, microelectronics applications, a highly efficient thinly applied coating is most desirable. In other applications, for example, heavy industrial applications, it is particularly important for such coatings to provide good insulating properties under extreme conditions such as in transformer coils found in power distribution transformers. U.S. Pat. No. 4,481,258 issued to Sattler et al. discloses the use of paper as insulating material. Although Sattler proposes a UV-curable coating be used as insulating material, it fails to disclose a coating with properties which are sufficient to meet the requirements of an insulator in extreme conditions. The coatings proposed in Sattler are certain UV-curable materials comprising acrylate-ester adducts, acrylate urethane adducts and acrylate functional diluents. These coatings require both UV cure and an additional thermal post-cure at a temperature of 130xc2x0 C. for 4-17 hr. The use in transformer coils of the coatings and processes disclosed in Sattler is unattractive, in particular because of the post-cure required. Because the coating of Sattler fails to provide an acceptable substitute for conventional materials used in transformers, paper insulation materials are still being used in the manufacture of transformer coils.
Effective insulating coatings, under the most rigorous environmental conditions such as, for example, high power transformer coils, should exhibit the desirable properties described below.
As the electrical conductor is coated and thereafter is bent in a required form, the cured coating should be flexible so that it can withstand bending of the coated conductor as it is wound into a coil.
The cured coating should be able to withstand immersion in oil for 28 days at 150xc2x0 C. as described in the aforementioned U.S. Pat. No. 4,481,258.
The cured coating should remain adherent at elevated temperature that is encountered when the transformer is under load.
The cured coating should have a dielectric constant smaller than 5 at 60 Hz (24xc2x0 C.).
The cured coating should have a dielectric dissipation factor smaller than 0.05 at 24xc2x0 C. before and after hot oil exposure and smaller than 0.5 at 150xc2x0 C., both at 60 Hz.
It is an object of the present invention to provide dielectric radiation-curable coating compositions, which have the properties identified above, for use as insulating coatings for a wide variety of electrically conductive substrates.
It is further an object of the present invention to disclose a method of manufacture of dielectric radiation-curable coating compositions, which have the properties identified above.
The present invention relates to a dielectric radiation-curable coating composition which can be applied to an electrical conductor, the composition, after cure providing a coating of a thickness of about 2.5 xcexcm to about 500 xcexcm and preferably about 10 xcexcm to about 50 xcexcm, which cured coating has a dielectric dissipation factor (60 Hz, 24xc2x0 C.) of lower than about 0.05, the coating composition being formulated from the combination of the following pre-mixture ingredients:
(A) about 15 wt. % to about 80 wt. % of at least one UV or radiation-curable acrylate oligomer;
(B) about 1 wt. % to about 20 wt. % of at least one vinyl reactive diluent or ene reactive diluent;
(C) about 10 wt. % to about 80 wt. % of at least one acrylate monomer diluent;
(D) about 0.5 wt. % to about 10 wt. % of at least one thiol compound, and
(E) optionally at least one additional additive, wherein the pre-mixture ingredients correspond to the identity of radiation-curable composition components prior to mixture with other ingredients.
Furthermore, the invention relates to a method of manufacturing an insulating radiation-cured coating on an electrical conductor comprising the steps of:
(A) coating an electrical conductor with a radiation-curable coating composition, the coating composition being formulated from the following premixture ingredients:
(i) about 15 wt. % to about 80 wt. % of at least one UV or radiation-curable acrylate oligomer;
(ii) about 1 wt. % to about 20 wt. % of at least one vinyl reactive diluent or ene reactive diluent;
(iii) about 10 wt. % to about 80 wt. % of at least one acrylate monomer diluent;
(iv) about 0.5 wt. % to about 10 wt. % of at least one thiol compound, and
(v) optionally at least one additional additive,
wherein the pre-mixture ingredients correspond to the identity of radiation-curable composition components prior to mixture with other ingredients; and
(B) exposing the electrical conductor coated with said coating composition to an effective amount of radiation to sufficiently cure the coating composition to form an insulating radiation-cured coating on the electrical conductor, wherein said insulating radiation-cured coating has a dielectric dissipation factor at 60 Hz at 24xc2x0 C. of lower than about 0.05 and a dissipation factor at 60 Hz at 150xc2x0 C. of lower than about 0.5 and an elongation at 25xc2x0 C. of a 25 xcexcm thick coating of greater than about 50%.
The present invention provides for the production of an improved insulating radiation-curable coating for an electrical conductor, the cured coating demonstrating strong adhesion to the surface of the electrical conductor at ambient conditions as well as after exposure to 150xc2x0 C. oil.
Good adhesion of the insulating radiation-curable coating to a wide variety of materials which can be used as electrical conductors, is desirable. Acrylate oligomers are known in field of adhesives. According to the invention, co-polymerization of thiol and vinyl compounds with urethane acrylates creates a urethane-acrylate-thiol-ene hybrid coating with superior adhesive properties to urethane acrylate coatings lacking the thiol-ene system. The present invention provides a radiation-curable coating composition with good adhesion to the underlying electrically conductive substrate while providing superior insulating qualities. This is important for long lasting bonding between the insulating coating and the underlying substrate under adverse environmental conditions, particularly elevated temperatures and humidity levels, the creation of hybrid acrylate-thiol-ene adhesive formulations is a marked improvement over the current methodology.
Co-pending U.S. patent application Ser. No. 08/961,084 filed Oct. 30, 1997 discloses the use of radiation-curable compositions for use as electrical conductor insulating coatings, the complete disclosure of which is incorporated herein by reference. Co-pending U.S. patent application Ser. No. 09/048,981, filed Mar. 27, 1998, discloses the use of thiol-ene systems as adhesives, the complete disclosure of which is incorporated herein by reference.
The thiol-ene systems of the present invention appear to allow copolymerization of vinyl moieties with acrylate moieties. In the absence of thiols, copolymerization of, for example, N-vinyl compounds is slow. Thiol compounds act as chain transfer agents, which may increase cure speed. Thiol chain transfer also facilitates co-polymerization of acrylate and vinyl compounds usually hindered in the absence of thiol. Co-polymers of vinyl and acrylate have increased flexibility and elongation (compensate for film shrinkage upon cure), which is essential for use with flexible substrates. Mechanisms of thiol chain transfer are described in Takeishi, J. Polym. Sci., 27:301-305 (1989) and Kirsh, Polym. Sci., 35(2): 98-114 (1993), the entire contents of which are incorporated herein by reference.